This invention relates to water-absorbing polyurethaneurea foams that contain no toxic leachable additives, and yet have the desirable physical properties commonly conferred by surfactants or other additives. The inventors have found that fine-celled, even-textured and reasonably hydrophilic foams can be prepared by incorporating high molecular weight, isocyanate-terminated prepolymers. These prepolymers function in the present invention to stabilize the cell structure of the foam, and to confer a hydrophilic character. Normally, a surfactant is added to confer such properties. The high molecular weight prepolymers are not known to be surfactants.
These foams are especially useful to make medical implants or wound care products because they are potentially safer for the patient.
Polyurethane foams are materials which can be used in diverse items such as coatings, absorbent pads and electronic component packaging. Naturally, the different applications can require different physical properties. In the medical field, for example, it is highly desirable to maximize the safety of any material which can come into contact with a patient's tissues and bodily fluids. In this case, the focus is on an effort to produce a foam which has the highly desirable fine-celled texture and hydrophilic nature of known foams, but which contains no potentially toxic or irritating additives.
Although the known foams appear to function very well in medical applications, the body is known to be a very aggressive environment. Thus the possibility exists that any nonbound additive contained in the foam might be leached out. In some circumstances this can be a useful feature, as in the metered dosing of a medicament over a period of time. However, it is preferable that this feature is controlled, and that the presence of potentially irritating components be minimized.
It is generally accepted in the polyurethane foam art that surface-active materials are essential ingredients in the manufacture of polyurethane foams (Woods, G., The ICI Polyurethanes Book, London: imperial Chemical Industries and John Wiley & Sons, Ltd., 1978, p. 48 ISBN 0471 914266. They help to control the size of the foam cells by stabilizing the gas bubbles formed during nucleation. In flexible foam manufacture, surfactants also help to control the degree of cell opening and increase the operating margin between the extremes of foam collapse, when cell opening occurs before the reaction mixture has sufficiently polymerized beyond its "gel point".
The production of hydrophilic polyurethane foams without surfactant is known and is described in the literature. However, it is known to all those who work with polyurethaneurea polymers that foams made without surfactants have many coarse cells, have little uniformity in cell size, and have very hydrophobic character. Furthermore, the inventors have found that known non-cytotoxic surfactants generally result in reticulated cell structures and medium to very coarse cell sizes. It is very desirable to make fine-celled, hydrophilic foams without surfactants or by using approved non-cytotoxic surfactants while having very fine cells and achieving very soft foams. This has not been achieved in hydrophilic foams made using either the one-shot or prepolymer routes.
The use of surface-active agents in polyurethane-urea foams is taught to be critical to the attainment of many useful properties (See U.S. Pat. No. 4,160,076, issued Jul. 3, 1979 to Guthrie). U.S. Pat. No. 3,890,254, issued Jun. 17, 1975 to Guthrie also teaches the use of particular surfactants, in conjunction with particular blowing agents, to make fully reticulated foams.
For hydrophilic foams made using an excess of (reactive) water, the presence of surfactants, even if hydroxy-functional, generally does not affect the progress of the reaction. Water competes with the hydroxyl groups in the isocyanate reaction, and there is no assurance that the hydroxyl groups on the surfactant will react at all. Because they do not take part in the chemical reaction, these surfactants are not bound into the structure of the foam. For that reason, the surfactants might be easily leached out of the foam under the right conditions. The inventors have found that certain isocyanate-capped high molecular weight molecules can be used instead of the non-bound surfactants to produce hydrophilic polyurethaneurea foams with uniform open or reticulated cell structure.
These isocyanate-capped species react into the foam and cannot be leachable when the foam is used. This finding was especially unexpected, because it was generally felt that reactive molecules would not be sufficiently mobile to orient at the polymer-air interface and have a cell-stabilizing action.
It was not expected that a high molecular weight, isocyanate-terminated, ethylene oxide-rich prepolymer would show cell-stabilizing action, because by itself it is not known to be a surfactant.
It was not expected that small amounts of these isocyanate-terminated polyols could modify the action of existing surfactants and allow the use of nocytotoxic surfactants to produce fine-celled, hydrophilic foams. The known non-cytotoxic surfactants are generally used to make coarse, reticulated cell structures in polyurethaneurea foams.
It is therefore an object of this invention to produce water-absorbing polyurethaneurea foams, from prepolymers, that contain no toxic surfactants or related leachable additives. These foams may be especially desirable for skin-contacting applications, such as cosmetic pads or wound care products, where freedom from toxic or irritating additives would be advantageous. Also, foamed materials for medical or implant applications benefit from having no toxic additives.
Other objects and advantages of this invention will be illuminated by the disclosure herein.